At that, I wonder what "walking the line" is about. I'd thought it meant living totally freely, "to the fullest", sort of on the line between life and death. Apparently it means quite the opposite though!: Yes, these expressions both mean to "behave," or take care not to deviate from the "straight and narrow."

... Listening to the Johnny Cash song, I have some other ideas, but I'll leave it to you if you wish to respond.